How Chillers Work
Air-Cooled Water Chillers – How They Work
Our air-cooled chillers are vapour-compression systems. The main parts of this type of system are the compressor, condenser and expansion valve, as well as the evaporator itself. This type of unit achieves its cooling through a refrigeration cycle.
It starts with a cool, low-pressure mix of liquid and vapour refrigerant that enters the evaporator. After this, it absorbs the heat from the warm water the chiller is cooling. This boils the liquid refrigerant in the evaporator, causing vapour to pull into the compressor.
The refrigerant is compressed to a high temperature and pressure, which is high enough to let the condenser give up its heat to the cooler air.
This causes the refrigerant to condense into a liquid, which then further sub-cools. It leaves the condenser and the high pressure liquid enters the expansion valve. This leads to a large pressure drop across the circuit.
The pressure reduction also causes a small part of the refrigerant to boil off, or flash. This will be seen in the site glass. It also shows if the chiller is short of gas. If this is the case, it flashes more. The boiled refrigerant helps cool the rest of the refrigerant to the desired temperature, before the mix enters the evaporator. And the cycle starts again.
Image: A scroll chiller compressor
Air Cooled Chiller Condensers – How They Work
The condenser is a key part of a chiller. It acts as a heat exchanger that rejects heat from the chiller’s refrigerant to air. A water-cooled chiller gives up heat into cool water from a cooling tower, an air-blast cooler or adiabatic cooler. Meanwhile, a standard condenser will use axial fans (propeller type) to draw air over a finned tubed heat transfer surface (heat exchanger).
The difference in temperature – or “delta T” – between the hot gas in the condenser and cooler outdoor air leads to the heat transfer. The heat reduction of the vapour will cause it to condense into liquid. The last part is the sub-cooler. This lowers the liquid gas temperature still further until it is under its condensing temperature. These units are also ideal when you need quick delivery and installation with the least fuss.
Image: Axial fans on top of a chiller
Chiller Evaporators – How They Work
This is a heat exchanger that transfers heat from a process or air conditioning water circuit to the cooler liquid refrigerant. Most air-cooled chillers come with a shell and tube evaporator, as well as a plate heat exchanger or a coil in tank evaporator.
The shell and tube evaporator is mainly used for chilled water applications. If a chiller has a shell and tube evaporator, the cool liquid evaporator flows through tubes within a shell. The circuit water fills the shell round the tubes. And, as the heat transfers from the water to the refrigerant, the gas boils in the tubes. This draws the resulting vapour into the compressor.
Hot water goes into the shell at one end, while the chilled water leaves at the other end. You can use a plate heat exchanger evaporator for chilled water or fluid cooling uses. Stainless steel evaporators, meanwhile, are well-suited to food and drink uses such as batch cooling or portable water systems.
A coil in tank model has a coil block – usually copper – and aluminium fins to provide an efficient heat transfer surface. The coil block is submerged in a chilled water tank, which acts as a thermal buffer. This type of coil works best for process cooling – such as reactor cooling systems in chemical cooling etc.
Summary of Chiller System Terms
- Air-cooled condenser – where the refrigerant flows through the tubes and rejects heat into a flow of ambient air. Most chiller hire units will come with these.
- Capillary Tube – fitted on small capacity chillers, with a long tube to reduce the pressure of the refrigerant.
- Centrifugal Fan – A type of fan that lets you fit ductwork onto the hot side of a condenser. Meanwhile, the design of the fan means it can work against a static pressure.
- Compressor – increases the pressure of the vapour, as well as the temperature
- Condenser – where vapour turns to liquid as it rejects heat.
- Cooling Tower – used for process cooling and water cooled chiller condenser cooling.
- Distributor – sends uniform gas supply through a submerged coil in a tank chiller evaporator.
- Enthalpy – The property of a refrigerant indicating its heat content per KG of refrigerant.
- Evaporator – where cool liquid refrigerant absorbs heat from the chilled water circuit.
- Expansion valve – maintains the pressure difference between the high and low pressure sides of the system.
- Fill – the heat transfer surface inside a cooling tower.
- Flash – the process of refrigerant being vaporised by a sudden loss of pressure when entering the low pressure side of the system.
- Hot Gas Muffler – lowers noise and vibration in reciprocating compressors.
- Liquid Line Filter Drier – removes moisture and foreign matter, while protecting the compressor.
- Pump Down Cycle – control sequence that pumps the refrigerant from the low pressure side of the system to the high pressure side.
- Shell & Tube Evaporator – where refrigerant flows through the tubes, while chilled water fills the surrounding shell.
- Shut Off Valve – isolates one part of the system from the rest.
- Sub-Cooler – The lower portion of the chillers condenser that further cools the saturated liquid refrigerant.
- Suction Header – collects the vapour when it leaves the tubes of a submerged coil evaporator.
- Suction Line Filter – removes foreign matter from the refrigeration system
- Superheat – the heat added to the refrigerant after it has vaporised in the evaporator.
- Water Cooled Condenser – uses water to remove heat from the refrigerant; most often a shell and tube type design.